Color-plus-clear coating systems involving the application of a colored or pigmented basecoat to a substrate followed by the application of a transparent or clear topcoat to the basecoat have become conventional as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image, and the clearcoat is particularly important for these properties.
Aminoplast-cured coating systems are also well known and provide many excellent coating properties. However, it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by acid. Conventional coating systems that contain hydroxyl functional film-forming resins and aminoplast crosslinking agents rely on a cure mechanism wherein hydroxyl groups on the resin react with the aminoplast to form ether linkages. See, for example, European Patent Application 0 257 848. Although not intending to be bound by any theory, it is believed that such ether linkages are vulnerable to acid attack and hence yield coatings with poor acid etch resistance.
Because many geographic areas encounter acidic precipitation, acid resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings. Hydroxyl-aminoplast coating systems of the prior art are not highly effective for providing protection against etching caused by acid rain.
It is desirable, therefore, to provide a color-plus-clear coating system which avoids the problems of the prior art by demonstrating improved acid etch resistance properties.